This is a replacement HPI Gearbox Spacer Set, and is intended for use with the HPI WR8 and WR8 Flux vehicles, as well as the Bullet family of vehicles. Install these shims next to the large differential output bearings to improve the tolerance between the differential ring and pinion gears. This will allow you to move the ring gear closer or farther away from the pinion gear, as needed to improve the gear mesh to provide increased durability and overall drivetrain efficiency.